The present invention relates to an improved process for bleaching pulp in an elemental-chlorine-free (ECF) bleaching process which reduces the volume of bleach plant effluent as well as the amount of adsorbable organic halides (AOX), chemical oxygen demand (COD) and color content of the effluent.
The process of bleaching pulp for use in papermaking may be performed with halogen-containing or non-halogen-containing bleaching agents. Currently, the industry has been moving away from halogen-containing agents due to public perception and environmental concerns over chlorinated organics and dioxins believed to be by-products of conventional halogen-based bleach processes. However, attempts to reduce the formation of organic halogen compounds during the bleaching process by use of peroxide and/or non-halogen oxygen-containing compounds often fall short of providing an economical pulp with sufficient brightness, viscosity and yield.
Many of the recently proposed techniques for reducing discharge of AOX compounds adversely affect the quality of the bleached product and economics of bleach plant operations. For example, extended delignification, totally chlorine-free (TCF) and totally effluent-free (TEF) bleaching processes result in an economic penalty and loss of product quality. The TCF bleaching process is capital-intensive and requires the addition of pressurized oxygen and/or ozone stages. Extended delignification to a Kappa No. of 15-20 typically needed for successful TCF bleaching causes excessive strength and yield loss leading to inferior product quality and higher operating costs. Extended delignification also exerts an additional load on the pulp mill chemical recovery system and caustic plant operations which have a negative impact on mill production.
Much recent work has focused on use of chlorine dioxide as a bleach agent as opposed to elemental chlorine due to the fact that chlorine dioxide, although generally less effective than elemental chlorine, offers the prospect of reduced chlorinated organics in the effluent with acceptable pulp properties. However, continued pressure for development of lowered effluent discharge has raised questions about whether use of chlorine dioxide alone represents a long-term solution to the problem.
Other techniques require the expenditure of significant capital for plant modifications in order to treat the effluent. There remains, therefore, a need for a cost effective means to substantially reduce the pulp bleaching filtrate volume as well as lowering the amount of AOX, COD and color in the effluent from the bleach plant while at the same time maintaining high brightness, viscosity and yield values of the bleached pulp.
Accordingly, it is an object of the present invention to provide a process which reduces the amount of chlorinated organics and other undesirable components in the effluent from a pulp bleaching process.
It is another object of the invention to provide a halogen-based bleaching process wherein the efficiency of the bleaching process is maintained without adversely affecting the pulp brightness, viscosity or yield while achieving significant reductions in the AOX, COD and color of a bleach plant effluent.
A further object of the invention is to provide a method for reducing the amount of halogen-based bleach agents needed in a pulp bleaching process to obtain a target brightness, viscosity and yield for the bleached pulp.
Another object of the invention is to reduce the volume of filtrate from the bleaching process which must be treated or recycled.
With regard to the above and other objects, the present invention provides a method for treating a pulp containing lignocellulosic fibers with chlorine dioxide wherein the adsorbable organic halide content of the bleached pulp effluent is significantly reduced. The method comprises bleaching the pulp in a EOD1EOPD2 or EOPD1EOPD2 bleaching sequence wherein pulp from at least one of the D1 or D2 stages is washed after the stage to produce a D0 or D filtrate and the D0 or D filtrate is recycled, mixed and treated with the pulp entering the initial EO or EOP stage. In the bleaching sequence, xe2x80x9cExe2x80x9d represents an extraction stage reinforced with oxygen and/or peroxide (EO or EOP), D1 represents an initial chlorine dioxide bleaching stage and D2 chlorine dioxide final bleaching stage. It is to be understood that the terminology xe2x80x9cextraction stagexe2x80x9d is not used herein in with regard to the initial EO or initial EOP stage according to the meaning often associated with the same; i.e., a treatment stage employing caustic following an acidic chlorination stage. Rather, as used herein regarding the initial EO or initial EOP stage, the word xe2x80x9cextractionxe2x80x9d refers to a stage in which the pulp is treated substantially according to the conditions of what is conventionally known as an extraction stage, i.e., treatment of the incoming pulp slurry with caustic (in this case supplemented with either oxygen or oxygen and peroxide) resulting in an alkaline pulp mixture, irrespective of whether the stage immediately follows a chlorine-containing bleaching stage.
Quite surprisingly, it has been found that essentially simultaneous treatment of recycled filtrate streams and delignification of the pulp may be obtained in an oxygen or oxygen and peroxide-assisted first extraction stage without adversely affecting the viscosity or brightness of the bleached pulp. Furthermore, the benefits of the present invention may be obtained with a significant reduction in the amount of organic halide compounds in the pulp and/or effluent produced by the bleaching process. These results are truly remarkable since they do not require significant changes in the bleaching process and thus may be implemented without substantially increasing the complexity or operating costs of the process and without the need for substantial capital expenditures.
Unlike the extended delignification processes, the effluent from first extraction stage according to the invention is not recycled to a recovery boiler for treatment, hence the use of filtrate from a D1 or D2 stage washer can be used to adjust the consistency of the pulp in the initial EO or EOP stage. Furthermore, much lower operating pressures are used in the initial EO or EOP stage according to the invention thereby lowering capital equipment costs.
In another aspect the invention provides a process for treating a pulp containing lignocellulosic fibers. The process comprises providing a lignocellulosic pulp at a consistency in the range of from about 20% to about 40% and at a pH in the range of from about 3 to about 11 and bleaching the pulp with an EOD1EOPD2 or EOPD1EOPD2 bleaching sequence wherein the pulp from at least one of the D0 or D stages is washed after the stage to produce a D1 or D2 filtrate and the D1 or D2 filtrate is recycled, mixed and treated with the pulp entering the initial EO or EOP stage.
In a preferred embodiment, the invention provides a process for treating a pulp containing lignocellulosic fibers with elemental chlorine-free bleaching agents in an EOD1EOPD2 or EOPD1EOPD2 bleaching sequence wherein the pulp has an initial Kappa number of greater than about 25 for softwood and greater than about 15 for hardwood and a consistency within the range of from about 25 to about 30%. Essentially simultaneous treatment of the filtrate from the first D1 stage may be obtained by recycling the filtrate from the first D1 stage of the bleaching sequence to the initial EO or EOP stage for use in adjusting the pulp consistency to a consistency in the range of 5 to 10% and/or as a portion of the wash liquid for the pulp washer after the first alkaline extraction stage.
One significant advantage of the bleaching process according to the invention is that it requires essentially no incremental chemical recovery equipment expenditures or additional caustic capacity, yet should meet or exceed the proposed EPA limits of nondetectable dioxins and polychlorophenolics and 0.156 kg/ADT of AOX in the bleach plant effluent. Both the EPA""s best available technology (BAT) and the bleach filtrate recycle process, such as the one under development by Champion International, using an ODEOPD bleaching sequence may meet the proposed EPA limits, but require high capital investment and additional chemical recovery and caustic plant capacity to accommodate the additional solids produced by oxygen delignification. Furthermore, the ODEOPD process does not recycle D stage filtrate to the pulp entering the oxygen delignification stage.
A further significant advantage of the bleaching process according to the invention is that it achieves reduced AOX, color and COD with significantly reduced filtrate volumes. Hence, the amount of effluent which needs to be treated in the plant effluent treatment system prior to discharge of treated water from the plant is significantly reduced.